The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI). The HCCI combustion mode comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical engine operating in HCCI combustion mode, the cylinder charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. The typical engine operating in the HCCI combustion mode can further operate using stratified charge fuel injection to control and modify the combustion process, including using stratified charge combustion to trigger the HCCI combustion. Because auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low nitrous oxides (NOx) emissions. The fuel/air mixture for auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy. The HCCI engine can operate at stoichiometry with substantial amounts of exhaust gas recirculation (EGR) to achieve effective combustion.
At medium engine speeds and loads, a combination of valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate thermal energy to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an engine in the auto-ignition combustion mode has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions. The benefits of auto-ignition combustion have been known for many years. The primary barrier to product implementation, however, has been the inability to control the auto-ignition combustion process, i.e., combustion phasing and rate of combustion. Late phasing or very slow combustion will result in partial burns and even possibly misfires. Too early phasing or too rapid combustion will lead to knock.
There is no direct control of start of combustion for an engine operating in the auto-ignition mode, as the chemical kinetics of the cylinder charge determine the start and course of the combustion. Chemical kinetics are sensitive to temperature and, as such, the controlled auto-ignition combustion process is sensitive to temperature. An important variable affecting the combustion initiation and progress is the effective temperature of the cylinder structure, i.e., temperature of cylinder walls, head, valve, and piston crown. Additionally, spark-assisted ignition is known to facilitate combustion in certain operating ranges.
Operation within an HCCI mode at higher loads is problematic, as energy present within the combustion chamber increases with increasing load. This increasing energy, exhibited for example by higher temperatures within the air fuel charge being combusted, increases likelihood of the air fuel charge combusting before the intended combustion point, resulting in an undesirable pressure wave or ringing from the combustion chamber.